The Elements of Geology eBook

Over a long belt which reaches from Wyoming across
Colorado into New Mexico no Triassic sediments are
found, nor is there any evidence that they were ever
present; hence this area was high land suffering erosion
during the Triassic. On each side of it, in eastern
Colorado and about the Black Hills, in western Texas,
in Utah, over the site of the Wasatch Mountains, and
southward into Arizona over the plateaus trenched
by the Colorado River, are large areas of Triassic
rocks, sandstones chiefly, with some rock salt and
gypsum. Fossils are very rare and none of them
marine. Here, then, lay broad shallow lakes often
salt, and warped basins, in which the waste of the
adjacent uplands gathered. To this system belong
the sandstones of the Garden of the Gods in Colorado,
which later earth movements have upturned with the
uplifted mountain flanks.

The Jurassic was marked with varied oscillations and
wide changes in the outline of sea and land.

Jurassic shales of immense thickness—­now
metamorphosed into slates—­are found infolded
into the Sierra Nevada Mountains. Hence during
Jurassic times the Sierra trough continued to subside,
and enormous deposits of mud were washed into it from
the land lying to the east. Contemporaneous lava
flows interbedded with the strata show that volcanic
action accompanied the downwarp, and that molten rock
was driven upward through fissures in the crust and
outspread over the sea floor in sheets of lava.

TheSierradeformation. Ever since
the middle of the Silurian, the Sierra trough had
been sinking, though no doubt with halts and interruptions,
until it contained nearly twenty-five thousand feet
of sediment. At the close of the Jurassic it yielded
to lateral pressure and the vast pile of strata was
crumpled and upheaved into towering mountains.
The Mesozoic muds were hardened and squeezed into
slates. The rocks were wrenched and broken, and
underground waters began the work of filling their
fissures with gold-bearing quartz, which was yet to
wait millions of years before the arrival of man to
mine it. Immense bodies of molten rock were intruded
into the crust as it suffered deformation, and these
appear in the large areas of granite which the later
denudation of the range has brought to light.

The same movements probably uplifted the rocks of
the Coast Range in a chain of islands. The whole
western part of the continent was raised and its seas
and lakes were for the most part drained away.

TheBritishIsles. The Triassic
strata of the British Isles are continental, and include
breccia beds of cemented talus, deposits of salt and
gypsum, and sandstones whose rounded and polished
grains are those of the wind-blown sands of deserts.
In Triassic times the British Isles were part of a
desert extending over much of northwestern Europe.